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described below. For sulfate, the scientific evidence was insufficient to set an AI. However, sulfate needs are met by the current recommended intakes for sulfur amino acids, which provide most of the inorganic sulfate needed for metabolism.

NUTRIENT FUNCTIONS AND THE INDICATORS USED TO ESTIMATE REQUIREMENTS

Water

The largest single constituent of the human body, water, is essential for cellular homeostasis and life. It provides the solvent for biochemical reactions, is the medium for material transport, and has unique physical properties (high specific heat) to absorb metabolic heat. Water is essential to maintain vascular volume, to support the supply of nutrients to tissues, and to remove waste via the cardiovascular system and renal and hepatic clearance. Body water deficits challenge the ability of the body to maintain homeostasis during perturbations (e.g., sickness, physical exercise, or climatic stress) and can impact function and health. Total water intake includes drinking water, water in other beverages, and water (moisture) in food (Table S-1).

Although a low intake of total water has been associated with some chronic diseases, this evidence is insufficient to establish water intake recommendations as a means of reducing the risk of chronic diseases. Instead, an AI for total water is set to prevent deleterious (primarily acute) effects of dehydration, which include metabolic and functional abnormalities.

Hydration status, as assessed by plasma or serum osmolality, is the primary indicator used for water. Physical activity and environmental conditions have substantial influences on water needs. Because of homeostatic responses, some degree of over- and underhydration can readily be compensated over the short term. While it might appear useful to estimate an average requirement (EAR) for water, it is not possible for a nutrient like water. Given the extreme variability in water needs that are not solely based on differences in metabolism, but also on environmental conditions and activity, there is not a single level of water intake that would ensure adequate hydration and optimal health for half of all apparently healthy persons in all environmental conditions. Hence, an EAR could not be established. Rather, an AI is established instead of an RDA, which must be derived from an EAR.

U.S. survey data from the Third National Health and Nutrition



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